This invention relates generally to manufacturing of Micro Electromechanical System (MEMS) devices, and more specifically to, manufacturing of a substrate layer for MEMS devices utilizing heavily doped silicon as an etch stop.
One method for making MEMS devices involves depositing a very heavily boron-doped silicon layer on a lightly doped silicon substrate wafer. After various patterning steps, part of the substrate is etched away in alkaline etchants such as potassium hydroxide or Ethylene-Diamine-Pyrocatechol (EDP), and water, plus a trace amount of Pyrazine. The heavily doped silicon layer is not affected by these etchants, creating a natural etch stop. In another method, the silicon wafer is bonded to a glass wafer. Prior to bonding, the silicon wafer can be patterned. Additional patterns can be made on the glass wafer. The entire lightly doped substrate is then etched away, leaving only the patterned, heavily doped layer attached to the glass. The boron dopant concentration in the doped layer is  greater than 1xc3x971020 cmxe2x88x923. At this concentration the boron atoms, which are smaller than silicon atoms, cause a shrinkage of the silicon lattice. Thus the doped layer has a high tensile strain compared to the substrate, causing the wafer to bow. The bow is severe enough that many pieces of fabrication equipment cannot handle the wafers. Therefore, additional layers or processes are required to control the wafer bow and create a relatively flat wafer. Two methods, boron-germanium co-doping and a backside tensile layer, have been widely used for controlling wafer bow. Germanium co-doping and backside tensile layering are described below in detail with respect to FIGS. 2 and 3 respectively.
There are two negative consequences of boron-germanium co-doping that make this approach unusable for some devices. One negative consequence is that the high germanium concentration ( greater than 1xc3x971021 cmxe2x88x923) degrades the mechanical properties of the silicon. An example of this is a high level of internal damping in a MEMS resonator. Another negative consequence is that the differing diffusion coefficients of boron and germanium in silicon result in some segregation at the interface between the substrate and the doped layer. This segregation creates undesirable stress gradients at the edge of the doped layer.
With regard to the backside tensile stress layer, when a heavily boron doped layer is deposited on a lightly doped substrate, the resultant wafer is heavily bowed as described above. A backside tensile stress layer balances the stress on the front side of the wafer, yielding a flat wafer. However, such a process requires more expensive, double-side polished substrates, more expensive, double-side deposition, more careful handling, and wafer preparation that must be done after epitaxial growth but before device fabrication can begin.
In one aspect, a method for controlling bow in wafers which utilize doped layers is provided. The method comprises depositing a silicon-germanium layer onto a substrate, depositing an undoped buffer layer onto the silicon-germanium layer, and depositing a boron doped layer onto the undoped layer.
In another aspect, a wafer is provided which comprises a substrate layer, a silicon-germanium layer deposited onto the substrate layer, an undoped buffer layer deposited onto the silicon-germanium layer, and a boron doped silicon layer deposited onto the undoped layer.
In still another aspect, a micro-electromechanical system is provided which comprises a housing and a micro-machine coupled to the housing. At least a portion of the micro-machine comprises boron-doped silicon that has been etched from a wafer which comprises a substrate layer, a silicon-germanium layer deposited onto the substrate layer, an undoped buffer layer deposited onto the silicon-germanium layer, and a silicon-boron layer deposited onto the undoped buffer layer.
In yet another aspect, a gyroscope is provided which comprises at least one proof mass, at least one motor drive comb, and at least one motor pick-off comb. The proof masses, motor drive combs, and motor pick-off combs comprise boron-doped silicon that has been etched from a wafer which comprises a substrate layer, a silicon-germanium layer deposited onto the substrate layer, an undoped buffer layer deposited onto the silicon-germanium layer, and a silicon-boron layer deposited onto the undoped buffer layer.
In another aspect, a method for reducing and controlling bow in wafers which are formed from stacked and doped silicon layers is provided. The method comprises creating stress-relieving dislocations within the stacked silicon layers.